In nutrient replete conditions, cancer cells can synthesize lipids, nucleic acids, and other macromolecules from extracellular glucose and glutamine. However, as aggressive lesions overgrow the capacity of existing tumor vasculature to perfuse them, cells must adapt to decreased availability of O2 and blood-born nutrients (i.e. glucose, amino acids, and lipids) to survive and proliferate. Collectively, we have discovered novel mechanisms whereby multiple cancer cell types adapt to metabolic stress. Specifically, neither glucose- nor glutamine-derived citrate can support de novo fatty acid synthesis in hypoxic tumor microenvironments, due to the O2- dependence of stearoyl-CoA desaturase (SCD1)-mediated generation of monounsaturated lipids. Instead, hypoxic cancer cells obtain lipids from exogenous sources, such as the blood stream or extracellular fluid. While it has long been appreciated that SCD1 catalyzes O2-consuming enzymatic reactions, the observation that growing tumors exhibit regions where O2 levels decline below levels supporting SCD1 activity is novel. This metabolic stress results in a maladaptive, cytotoxic Unfolded Protein Response (UPR) within the ER. In this Program, three integrated Projects will pursue the following specific aims to: Aim 1: Determine how Ras-transformed cells acquire exogenous unsaturated fatty acids; is macropinocytosis the relevant mechanism? Moreover, the role of autophagy components in Ras-mediated lipid utilization will be explored; Aim 2: Assess the impact of unsaturated: saturated fatty acid ratios on mTORC1- and pVHL-regulated cell survival under hypoxic conditions. Decreased levels of unsaturated fatty acids result in an UPR-mediated cell death, and this mechanism will be further explored in the lipogenic phenotype typical of ccRCC cells and primary patient samples; and Aim 3: Determine if PERK kinase inhibition represents an attractive therapeutic option for treating malignant melanoma. Apparently, incomplete PERK inhibition results in melanoma growth, while e 50% inhibition results in significant melanoma cell death. These findings are important because mutations in PERK functional domains have now been identified in human patient samples through TCGA efforts, including melanoma. Collectively, through these Projects and the Overall Program, we will define important new mechanisms by which cancer cells adapt to tumor microenvironmental stress, and determine whether these pathways can be targeted for anti-cancer treatment.